A muffler provided with an outer shell, an inlet pipe, an outlet pipe, and a valve is conventionally known; the inlet pipe lets exhaust gases flow from an internal combustion engine into the outer shell; the outlet pipe discharges the exhaust gases in the outer shell to the outside; and the valve changes a flow path of the exhaust gases inside the outer shell according to an operation status of the internal combustion engine (see Patent Document 1).
The outlet pipe described in Patent Document 1 is formed into a tubular shape including multiple through holes on an outer surface thereof as well as containing a pair of open end-parts; one open end-part and the through holes are arranged to be placed inside the outer shell. An opening of the outlet pipe arranged inside the outer shell is hereinafter referred to as an exhaust-gas inlet port.
The valve is provided with a valve body, an axis member to pivotally support the valve body, and a biasing member to apply a biasing force to the valve body; wherein the valve is closed by occluding the exhaust-gas inlet port with the valve body by the biasing force, if the operation status of the internal combustion engine is in low rotation, and the valve is opened by releasing the valve body from the exhaust-gas inlet port, if the operation status of the internal combustion engine is in high rotation.
In other words, the exhaust gases from the internal combustion engine flow into the outlet pipe from the through holes, if the operation status of the internal combustion engine is in low rotation; and flow into the outlet pipe from the exhaust-gas inlet port, if the operation status of the internal combustion engine is in high rotation.
It is favorable to form a general exhaust-gas inlet port with a diameter equal to or larger than that of the outlet pipe to smoothly discharge the exhaust gases, the amount of flow of which is increased because of a high rotational frequency of the internal combustion engine, to the outside. In order to dispose a valve to an exhaust-gas inlet port in such size, a muffler described in Patent Document 1 is configured to dispose a tubular diameter-expansion part with a diameter larger than that of the exhaust-gas inlet port (i.e., diameter of the outlet pipe) to an end part of the outlet pipe, and then to fix an axis member of the valve to that diameter-expansion part.